Optical coherence tomography (OCT) is an interferometric imaging technique described, for example, in U.S. Pat. No. 5,975,697. In order to produce OCT scans in the depth direction, the optical path length between a reference beam and object is varied, which requires a technique for progressively varying an optical path length.
Different depth scanning procedures have been devised to be incorporated into optical coherence tomography set-ups. One of the most successful procedures uses the introduction of a phase term linearly dependent on the optical frequency. The Fourier transformation leads to an equivalent optical path. The method is inspired from research on processing of femtosecond pulses, as described in the paper “400 Hz mechanical scanning delay line”, published in Opt. Letters, Vol., 18, No. 7, 1993, pages 558-560, by K. F. Kwong et al. The method has the added advantage of allowing for dispersion compensation. Known under the name of spectral delay line, the method uses a diffraction grating, a lens and a galvanometer scanner, as disclosed in U.S. Pat. No. 611,645, Grating based phase control optical delay line by, G. Tearney, E. Bouma, J. Fujimoto and by U.S. Pat. No. 6,421,164 by, G. Tearney, “Interferometeric imaging with a grating based phase control optical delay line”, U.S. Pat. No. 6,282,011B1. The linear variation of phase versus optical frequency is introduced by tilting a galvanometer mirror behind a lens, where the galvanometer mirror is situated in the Fourier plane of the lens. Tilting the mirror causes the returned beam to be deviated from the grating along a direction parallel to the incident beam. Usually, OCT configurations use reflective set-ups, where the same fiber aperture is employed in launching light towards and from the depth scanning device. This means that during scanning in depth, the amount of light re-injected back into the fiber aperture varies, i.e. the beam walks off the main direction (where the loss is at a minimum). This phenomenon is known as walk-off.
US patent publication no. 20030137669A1, by A. M. Rollins, “Aspects of basic OCT engine technologies for high speed optical coherence tomography and light source and other improvements in optical coherence tomography” describes a technique for reducing walk-off wherein a mirror is added to return the beam back to the scanning delay line, the beam is re-circulated four times via the diffraction grating which finally leads to a de-scanning of the lateral movement of the output beam.
Such configurations have been devised to operate in transmission as well, where a different fiber aperture is used to capture the output beam, as disclosed in the U.S. Pat. No. 6,564,089 B2, by J. A. Izatt, “Optical Imaging Device”. However, this transmissive scanning delay line descans the beam by four time diffraction off the diffraction grating which leads to losses of the optical beam. It will be desirable if the high loss element such as the diffracting grating is used less in the configuration.
It would also be desirable to combine optimally the scanning depth capability of such devices with that of transversally scanning the beam in order to obtain a versatility of scanning modes in generating OCT images and reconstruct the 3D volume of a scattering volume, such as that of a biologic or industrial sample.